1. Field of the Invention
This invention is in the field of microscopy and metrology, and specifically relates to heating a probe of a probe-based instrument to obtain stable measurements of a heated sample.
2. Description of the Related Art
A variety of probe-based microscopy and metrology instruments are known for obtaining metrology measurements and imaging of surface features to the Angstrom scale. These instruments include stylus profilometers and scanning probe microscopes (SPMs) including atomic force microscopes (AFMs). AFMs usually operate by creating relative motion between a probe and a sample surface using a high resolution three axis scanner while measuring the topography or some other surface property, as described in Hansma et al. in U.S. Pat. No. RE 34,489. AFMs typically include a probe, usually a very small cantilever fixed at one end with a sharp probe tip attached to the opposite, or free, end. The probe tip is brought very near to or into contact with a surface to be examined, and the deflection of the cantilever in response to the tip's interaction with the surface is measured with an extremely sensitive deflection detector, often an optical lever system such as described by Hansma et al, or some other deflection detector such as a strain gauge, capacitance sensor, or others well known in the art. Using piezoelectric scanners, optical lever deflection detectors, and very small probe cantilever arms fabricated using photolithographic techniques, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum. Additionally, AFMs can obtain both surface information and subsurface information.
Because of their resolution and versatility, AFMs are important measurement devices in a diversity of fields ranging from semiconductor manufacturing to biological research. One major application of AFMs is high resolution mapping of polymer structures. Atomic force microscopy has made it possible to obtain information about the morphology and nanostructure of polymer materials, and has also made it possible to correlate the fine structural arrangement of the polymer with other properties, such as mechanical properties. Another major application of AFMs is compositional mapping of the distribution of polymer components. Atomic force microscopy has been utilized to obtain information about the extent to which different polymer components are perfectly blended when forming polymer blends.
Early AFMs operated in what is commonly referred to as contact mode by scanning the tip in contact with the surface, thereby causing the cantilever to bend in response to the sample features. Typically, the output of the deflection detector was used as an error signal to a feedback loop which servoed the vertical axis scanner up or down to maintain a constant preset cantilever deflection as the tip was scanned laterally over the sample surface topography. The servo signal versus lateral position created a topographic map (or image) of the sample surface. Thus, the AFM maintained a constant cantilever deflection and accordingly a constant force on the sample surface during lateral scanning. Using very small, microfabricated cantilevers, the tip-sample force in contact mode AFMs could be maintained at force levels sufficiently small to allow imaging of biological substances, and in some cases achieve atomic resolution. In addition, an AFM can measure very small force interactions between the tip and sample. By suitably preparing the probe, such as coating it with an appropriate material, other parameters such as magnetic or electric fields may be measured with an AFM.
Contact mode is basically a DC measurement. Other modes of operation have been developed in which the cantilever is oscillated. At this time, three oscillating probe modes are common in AFMs, non-contact (Martin, et al., J. Applied Physics 61(10) May 15, 1987), shear force (U.S. Pat. No. 5,254,854 by Betzig et al), and Tapping.TM. or TappingMode.TM. ("Tapping and TappingMode" are trademarks of Veeco Instruments, Inc.). U.S. patents relating to Tapping and TappingMode include U.S. Pat. Nos. 5,226,801, 5,412,980 and 5,519,212, by Elings et al. These modes, particularly TappingMode, have become extremely commercially successful. An improved mode of operation which has also become extremely commercially successful is light Tapping, which is disclosed in U.S. patent application Ser. No. 08/984,058 (allowed May 10, 1999).
In oscillating AFM operation, the probe is oscillated, typically at or near the probe's resonant frequency, and brought near the sample surface. In all oscillation modes the effect of the surface on probe oscillation is used as one of the signals of interest, either as an error signal for a feedback loop or as direct measure of tip-sample interaction. For example, many AFMs in an oscillating mode employ a feedback loop that uses changes in the oscillation amplitude due to interaction with the surface to maintain that amplitude substantially constant. In non-contact and TappingMode operations, the free oscillation is substantially perpendicular (AFM cantilevers are commonly at an 11-12 degree angle of declination) to the plane of the sample surface, while in shear force measurement operation, the free oscillation is essentially parallel with the plane of the sample surface. Non-contact operation relies on non-contact force gradients to affect the resonant properties of the probe in a measurable fashion, while TappingMode operation relies on the more robust interaction of actually striking the surface and losing some energy to the surface (see, e.g., U.S. patent application Ser. No. 08/898,469 by Cleveland, issued as U.S. Pat. No. 6,038,916). Mixed modes such as Tapping for topography measurements and non-contact for other force measurements, i.e., magnetic field, are commonly used (see, e.g., U.S. Pat. No. 5,308,974 by Elings et al.).
In addition to the modes described above, other modes of operation of an atomic force microscope are also possible. The invention can be used in all of these modes of operation. Additionally, although the invention will be described in the context of AFMs, it should be understood that the invention is applicable to other SPMs and to probe-based instruments in general. Therefore, for example, the invention may also be utilized in conjunction with stylus profilometers.
AFM measurements are commonly performed at ambient conditions. However, with the development of AFM applications, there has arisen a need to be able to perform AFM measurements at elevated temperatures. This is especially important for studies of polymer materials, whose structure and performance strongly depends on temperature because of the thermal phase transitions inherent to such materials.
In order to perform AFM measurements at elevated temperatures, it is known to use a sample stage heater to heat a sample while performing AFM measurements. Problems have been encountered when attempting to obtain measurements of heated samples, however, because condensation occurs on the cantilever of the AFM probe. Droplets ranging from a few microns to tens of microns in size and can be distinguished in an optical microscope. The droplets typically appear and change size at a broad range of temperatures depending on the volativity of compounds in the sample and the surrounding vapor pressure for those compounds, and are also attributable in part to condensed water or other volatiles from the atmosphere. Droplets of volatile compounds emitted from the heated sample also condense on the relatively cool probe. The reason for the condensation is that the temperature of the cantilever, which is ordinarily 10 to 15 microns away from the hot sample, is cooler than the sample. Consequently, traces of moisture that have been heated in the immediate vicinity of the hot sample condense on the cooler cantilever surface.
This condensation prevents sufficiently stable imaging for at least two reasons. First, the droplets significantly hamper measurements of probe deflection (in a contact mode of operation) or measurements of probe oscillation amplitude, phase or frequency (in an oscillation mode of operation). For example, in systems that use an optical detection scheme, the spherical shape of the condensed droplets leads to scattering of the laser beam which in turn leads to a reduction in the intensity or diffusion of the reflected laser beam that is detected by the detectors. Therefore, the signal to noise ratio of the measurement substantially decreases. Additionally, the intensity of the reflected laser beam also fluctuates because of the spontaneous appearance and disappearance of the water droplets. The formation of droplets may also change the electrical characteristics of cantilevers that are actuated or have their deflections measured by piezoelectric or other elements on the cantilever itself.
The second problem is that the droplets affect the effective physical properties of the cantilever. For instance, because the accretion of droplets effectively increases the mass of the cantilever, the resonant frequency of the cantilever decreases, and also becomes unstable due to the continuous changes in the mass of the cantilever associated with the spontaneous appearance and disappearance of the droplets. In practice, variations of about 400 Hz in the resonant frequency of the cantilever have been observed.
To remedy the condensation problem, attempts have been made to enclose the sample in a humidity-free atmosphere, such as a dry nitrogen atmosphere or vacuum. However, this approach does not completely eliminate the problem of condensation on the probe because, even when all of the water is eliminated from the atmosphere, droplets of volatile compounds from the heated sample still condense on the probe.
Other attempts have been made to continuously purge the sample atmosphere to keep the atmosphere dry. According to this approach, moisture associated with the evaporated volatile sample components is continuously removed from the sample chamber and replaced with dry nitrogen, for example. However, purging the atmosphere in this manner causes cantilever movement due to gas currents moving around it (like a flag waving in the wind) and therefore introduces additional noise during imaging. Due to problems of these types, stable AFM measurements at elevated temperatures have remained elusive.